1. Technical Field
The present invention relates generally to Time Domain Reflectometry Systems (hereinafter "TDR") used to measure the propagation velocity of RF pulses along transmission lines, or probes, inserted into material undergoing test for water or other liquid content, or to determine material levels in a container, or to determine material dielectric constants; and, more particularly, to improved transmission line probes and/or probe adapters employing at least one and, preferably two or more, remotely operated, active, normally open, variable impedance devices for establishing one or more unambiguous large amplitude timing markers that are readily discernible and measurable by both observation and manual techniques, as well as by automated electronic processing, measuring and display systems.
More specifically, and in one preferred embodiment, the present invention--an invention which finds particularly advantageous, but by no means exclusive, use in the fields of soil science, hydrology, agriculture, seedling nurseries and similar soil/moisture environments--relates to methods and apparatus including active, as contrasted with passive, variable impedance devices such, merely by way of example, as at least one, and preferably n pairs (where "n" is any desired whole integer) of PIN diodes interconnecting the parallel transmission lines defining a moisture sensitive probe at selected, known, spaced points X.sub.1, X.sub.2 . . . X.sub.n, along the probe. As a consequence of this arrangement, when any given variable impedance device or diode is biased to the conductive state, the resulting electrical short produces an electrical discontinuity which serves to transmit a large amplitude reflection to the TDR instrument establishing an unambiguous, large amplitude, readily discernible and measurable timing marker T.sub.n ; yet, when the variable impedance device or diode is biased to the non-conductive or open state, the electromagnetic pulses are propagated down the transmission line without change except for attenuation inherently resulting from the natural impedance characteristics of the transmission line.
Stated differently, the remotely operated, active, variable impedance devices mounted on the probe, when biased to conduction to establish a shorted electrical discontinuity in the transmission line, serve to greatly increase the amplitude, and therefore the detectability and measurability, of the T.sub.1 , T.sub.2. . . T.sub.n reflections respectively produced at the X.sub.1, X.sub.2 . . . X.sub.n specific points along the probe where the variable impedance devices are located.
The invention finds particularly advantageous use when employed with differential detection apparatus and methods, including waveform subtraction techniques, so as to provide a significant increase in the effective amplitude of the reflections of interest, as well as significant reduction and/or elimination of background noise resulting from, for example, mismatched and inexpensive electrical components and other spurious discontinuities, such, merely by way of example, as spurious reflections from layer interfaces in layered soil.
Probes employing variable impedance devices may be interrogated, and differential detection techniques may be employed using a conventional TDR instrument; or, in another of its important aspects, the invention permits use of synchronous detection techniques for processing signals at one or more precise timing markers T.sub.1, T.sub.2 . . . T.sub.n where such signals are representative of probe reflections derived from shortable diodes, or similar shortable variable impedance devices, to provide remotely shortable diode ON/OFF modulation.
In another embodiment, this invention allows the optional processing of reflections in those instances where the probes do not have remotely shortable impedance device capability, or where the probes employ only a single remotely shortable impedance device. The present invention permits of time delay modulation by rapidly switching between two preset delay circuits so as to establish first and second preset time delays T.sub.A, T.sub.B and to generate a square wave output signal from a Sample-And-Hold circuit whose amplitude is proportional to the slope of the reflection, which signal is then processed using synchronous detection techniques. Such an arrangement is particularly advantageous when dealing with relatively homogeneous soils of the type found in seedling nurseries where the natural reflection at the end of the transmission line/probe at time T.sub.2 is typically large and free from distortion.
As the ensuing description proceeds, the invention will be described in connection with a TDR system for detecting and measuring soil water content and providing moisture profiles of the soil medium under test since the invention finds particularly advantageous application in this particular agricultural field and in the related fields of soil science and hydrology. However, those skilled in the art will appreciate as the ensuing description proceeds that the invention is not limited to measurement of soil water content and/or generating moisture profiles; but, rather, it will also find advantageous application in environments wherein the medium under test may comprise, for example, granular and/or particulate materials other than soil, sand or the like--for example, grain--and where the liquid whose volume content is of interest is other than water--for example, alcohol or the like. Moreover, it will be understood by those skilled in the art that the invention can also be used to determine levels of liquids or dry particulate solids in storage containers, or to determine the dielectric constant K of any solid material through which the probe extends. Therefore, it will be understood that terms such as "soil", "water" and "moisture" are used herein and in the appended claims in a non-limiting sense and for descriptive purposes only.
2. Background Art
Those skilled in the art will, of course, appreciate that TDR apparatus and methods have been widely used for many years in a wide range of different applications including, but not limited to, the measurement of soil water content and similar material liquid content. Such systems are based upon the principle that since the dielectric constant K of water is approximately 80--e.g., 78.9 at 23.degree. C.--while the dielectric constants for various materials are known and considerably lower--for example, the dielectric constant for most dry soil solids ranges from about 2 to about 5--a measurement of the dielectric constant of a soil or other material sample provides an excellent measure of the soil's or other material's water content or other dielectric characteristic. And, since it is also known that the apparent dielectric constant K.sub.a of a moist soil sample or other moist material sample is directly related to the propagation velocity V of an electromagnetic wave as it transits an RF transmission line extending through the particular sample undergoing test, TDR systems have been designed to provide fast rise time electromagnetic pulses which are propagated along a transmission line of known length while measuring the times of arrival T.sub.1, T.sub.2 of reflections from electrical discontinuities in the transmission line at two known spaced points X.sub.1, X.sub.2 --for example, where X.sub.1 represents the air/material interface where the coaxial connecting cable is attached to the transmission line probe and X.sub.2 represents the distal end of the transmission line probe, thereby enabling calculation of the propagation velocity V of the electromagnetic wave and, therefore, calculation of the apparent dielectric constant K.sub.a of the material under-going test and through which the transmission line probe extends. Such calculated apparent dielectric constant K.sub.a may be of interest, or it may provide a direct indication of the test material's water (or other liquid) content.
The foregoing general principles of TDR systems are, as stated above, well known and have been described in considerable detail in the literature. Those interested in a comprehensive, but far from exhaustive, compilation of said literature references are referred to the following articles:
______________________________________ Ref. No. 1 Alharti, A. and Lange, J., Soil Water Saturation: Dielectric Determination, WATER RESOURCES RESEARCH, Vol. 23, No. 4, pp. 591-595 (April, 1987). Ref. No. 2 Anon., Circuit Description, TEKTRONIX 1502 TDR INSTRUMENT OPERATORS AND MAINTENANCE MANUAL, Sect. 3, pp. 3-1- 3-6 and PULSER/SAMPLER DRG. (rev.) (January, 1986). Ref. No. 3 Baker, J. M. and Allmaras, R. R., System for Automating and Multiplexing Soil Moisture Measurement by Time-Domain Reflectometry, SOIL SCIENCE SOCIETY OF AMERICA JOURNAL, Vol. 54, No. 1, pp. 1-6 (January-February, 1990). Ref. No. 4 Baker, J. M. and Lascano, R. J., The Spatial Sensitivity of Time-Domain Reflectometry, SOIL SCIENCE, Vol. 147, No. 5, pp. 378-384 (May, 1989). Ref. No. 5 Dalton, F. N., Herkelrath, W. N., Rawlins, D. S. and Rhoades, J. D., Time-Domain Reflectometry: Simultaneous Measurement of Soil Water Content and Electrical Conductivity with a Single Probe, SCIENCE, Vol. 224, pp. 989-990 (1984). Ref. No. 6 Dalton, F. N. and van Genuchten, M. Th., The Time-Domain Reflectometry Method For Measuring Soil Water Content And Salinity, GEODERMA, Vol. 38, pp. 237-250 (1986). Ref. No. 7 Dasberg, S. and Dalton, F. N., Time Domain Reflectometry Field Measurements of Soil Water Content and Electrical Conductivity, SOIL SCIENCE SOCIETY OF AMERICA, Vol. 49, pp. 293-297 (1985). Ref. No. 8 Dasberg, S. and Hopmans, J. W., Time Domain Reflectometry Calibration for Uniformly and Nonuniformly Wetted Sandy and Clayey Loam Soils, SOIL SCIENCE SOCIETY OF AMERICA JOURNAL, Vol. 56, pp. 1341-1345 (1992). Ref. No. 9 Fellner-Feldegg, H., The Measurement of Dielectrics in the Time Domain, THE JOURNAL OF PHYSICAL CHEMISTRY, Vol. 73, No. 3, pp. 616-623 (March, 1969). Ref. No. 10 Grove, W. M., Sampling for Oscilloscopes and Other RF Systems: Dc Through X-Band, ISEE, TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, Vol. MTT-14, No. 12 (December, 1966). Ref. No. 11 Heimovaara, T. J. and Bouten, W., A Computer-Controlled 36-Channel Time Domain Reflectometry System for Monitoring Soil Water Contents, WATER RESOURCES RESEARCH, Vol. 26, No. 10, pp. 2311-2316 (October, 1990). Ref. No. 12 Hook, W. R., Livingston, N. J., Sun, Z. J. and Hook, P. B., Remote Diode Shorting Improves Measurement of Soil Water by Time Domain Reflectometry, SOIL SCIENCE SOCIETY OF AMERICA JOURNAL, Vol. 56, pp. 1384-1391 (September-October, 1992). Ref. No. 13 Kachanoski, R. G., Pringle, E. and Ward, A., Field Measurement of Solute Travel Times Using Time Domain Reflectometry, SOIL SCIENCE SOCIETY OF AMERICA JOURNAL, Vol. 56, pp. 47- 52 (1992). Ref. No. 14 Ledieu, J., de Ridder, P., de Clerck, P. and Dautrebande, S., A Method of Measuring Soil Moisture by Time- Domain Reflectometry, JOURNAL OF HYDROLOGY, Vol. 88, pp. 319-328 (1986). Ref. No. 15 Malicki, M. A. and Skierucha, W. M., A Manually Controlled TDR Soil Moisture Meter Operating With 300ps Rise-Time Needle Pulse, PROCEEDINGS OF INTERNATIONAL CONFERENCE ON MEASUREMENT OF SOIL AND PLANT WATER STATUS, Vol. 1-Soils, pp. 103-109, Academic Press, Inc. (1987). Ref. No. 16 Nadler, A., Dasberg, S. and Lapid, I., Time Domain Reflectometry Measurements of Water Content and Electrical Conductivity of Layered Soil Columns, SOIL SCIENCE SOCIETY OF AMERICA JOURNAL, Vol. 55, pp. 938-943 (July-August, 1991). Ref. No. 17 Rhoades, J. D., Raats, P. A. C. and Prather, R. J., Effects of Liquid- phase Electrical Conductivity, Water Content, and Surface Conductivity on Bulk Soil Electrical Conductivity, SOIL SCIENCE SOCIETY OF AMERICA JOURNAL, Vol. 40, pp. 651-655 (1976). Ref. No. 18 Rhoades, J. D. and van Schilfgaarde, J., An Electrical Conductivity Probe for Determining Soil Salinity, SOIL SCIENCE SOCIETY OF AMERICA JOURNAL, Vol. 40, pp. 647-651 (1976). Ref. No. 19 Roth, K., Schulin, R., Fluhler, H. and Attinger, W., Calibration of Time Domain Reflectometry for Water Content Measurement Using a Composite Dielectric Approach, WATER RESOURCES RESEARCH, Vol. 26, No. 10, pp. 2267- 2273 (October, 1990). Reg. No. 20 Topp, G. C., The Application Of Time- Domain Reflectometry (TDR) To Soil Water Content Measurement, PROCEEDINGS OF INTERNATIONAL CONFERENCE ON MEASUREMENT OF SOIL AND PLANT WATER STATUS, Vol. 1-Soils, pp. 85-93, Academic Press, Inc. (1987). Ref. No. 21 Topp, G. C., Davis, J. L. and Annan, A. P., Electromagnetic Determination of Soil Water Content: Measurements in Coaxial Transmission Lines, WATER RESOURCES RESEARCH, Vol. 16, No. 3, pp. 574-582 (June, 1980). Ref. No. 22 Topp, G. C., Davis, J. L. and Annan, A. P., Electromagnetic Determination of Soil Water Content Using TDR:II. Evaluation of Installation and Configuration of Parallel Transmission Lines, SOIL SCIENCE SOCIETY OF AMERICA JOURNAL, Vol. 46, pp. 672-678 (1982). Ref. No. 23 Topp, G. C. and Davis, J. L., Time- Domain Reflectometry (TDR) And Its Application To Irrigation Scheduling, ADVANCES IN IRRIGATION, Vol. 3, pp. 107-127, Academic Press, Inc. (1985). Ref. No. 24 Topp, G. C. and Davis, J. L., Measurement of Soil Water Content using Time-domain Reflectrometry (TDR): A Field Evaluation, SOIL SCIENCE SOCIETY OF AMERICA JOURNAL, Vol. 49, pp. 19-24 (1985). Ref. No. 25 Topp, G. C., Yanuka, M., Zebchuk, W. D. and Zegelin, S., Determination of Electrical Conductivity Using Time Domain Reflectometry: Soil and Water Experiments in Coaxial Lines, WATER RESOURCES RESEARCH, Vol. 24, No. 7, pp. 945-952 (July, 1988). Ref. No. 26 Wraith, J. M. and Baker, J. M., High- Resolution Measurement of Root Water Uptake Using Automated Time-Domain Reflectometry, SOIL SCIENCE SOCIETY OF AMERICA JOURNAL, Vol. 55, pp. 928- 932 (1991). Ref. No. 27 Yanuka, M., Topp, G. C., Zegelin, S. and Zebchuk, W. D., Multiple Reflection and Attenuation of Time Domain Reflectometry Pulses: Theoretical Considerations for Applications to Soil and Water, WATER RESOURCES RESEARCH, Vol. 24, No. 7, pp. 939-944 (July, 1988). Ref. No. 28 Zegelin, S. J., White, I. and Jenkins, D. R., Improved Field Probes for Soil Water Content and Electrical Conductivity Measurement Using Time Domain Reflectometry, WATER RESOURCES RESEARCH, Vol. 25, No. 11, pp. 2367- 2376 (November, 1989). Ref. No. 29 Zimmerman, A., The State Of The Art In Sampling, TEKTRONIX SERVICE SCOPE, No. 52, pp. 1-7 (October, 1968). ______________________________________
While TDR systems have been known and used for decades in non-agricultural fields such, for example, as in the telecommunication field, their application to agricultural fields and for use in measurement of soil water content and/or moisture profiles began to be seriously explored in or about the late 1960s and the early 1970s. Malicki et al., Ref. No. 13. Prior to that time, the more widely used systems and/or equipment for measurement of soil water content included, merely by way of example: i) neutron modulation or scattering; ii) neutron probes; iii) gamma attenuation; iv) gravimetric and thermogravimetric systems; v) lysimetry; vi) tensiometers; and vii), gypsum blocks. These conventional techniques, and some of the disadvantages inherent in their use, have been described in the literature. Baker et al., Ref. No. 3; Dalton et al., Ref. No. 5; and, Topp et al., Ref. No. 23. However, despite their known disadvantages, such conventional systems have continued to be utilized in the field as design, development and experimental work with TDR systems have progressed.
Use of TDR systems to measure, for example, soil water content has many advantages over the known conventional systems described above such, merely by way of example, as:
i) excellent spatial resolution (Baker et al., Ref. No. 4); PA1 ii) ability to measure close to the soil surface (Topp, Ref. No. 20; and, Zegelin et al., Ref. No. 28); PA1 iii) excellent multiplexing capability (Baker et al., Ref. No. 3; and, Zegelin et al., Ref. No. 28); PA1 iv) potential for accuracies greater than 2.times.10.sup.-2 m.sup.3 /m.sup.3 (Topp, Ref. No. 20; and, Topp, et al., Ref. No. 24); PA1 v) potential for rapid reading in the field with only minimal soil disturbance (Baker et al., Ref. No. 4; Kachanoski et al., Ref. No. 13; and, Roth et al., Ref. No. 19); and, PA1 vi) measurements of volumetric water content appear to be substantially independent of soil type and salinity for many, if not most, soil environments (Dasberg et al., Ref. No. 7; Fellner-Feldegg, Ref. No. 9; and, Topp et al., Ref. Nos. 21, 23 and 24). PA1 i) fast rise times for T.sub.1 and T.sub.2 reflections; PA1 ii) amplitudes for T.sub.1 and T.sub.2 reflections which are at least 90% of the maximum available amplitude--i.e., the amplitude of the reflection from the unterminated end of the coaxial connector cable; PA1 iii) minimal electromagnetic pickup; and, PA1 iv) minimal cost. PA1 i) U.S. Pat. No. 4,013,950-Falls [a probe for measuring electromagnetic impedance characteristics of soils]; ii) U.S. Pat. Nos. 4,281,285-Bastida and 4,341,112-Mackay et al. [the use of RF radiation to provide an indication of soil water content]; iii) U.S. Pat. No. 4,754,214-Bramanti et al. [a system for determining the amount of coal in furnace ash using a reflected microwave signal]; and iv), U.S. Pat. No. 4,807,471-Cournane et al. [a swept frequency system wherein the transmission line conductors are terminated by a passive variable impedance device such as a PIN diode for level measurement in storage silos]. PA1 i) the complexity and high cost of conventional TDR instrumentation and, particularly, high-performance TDR instruments; PA1 ii) the difficulties in detecting and accurately measuring relatively weak reflections of interest and/or distinguishing such reflections of interest from background noise; PA1 iii) poor signal-to-noise ratios inherent in most conventional TDR systems; PA1 iv) the unreliability of soil water content measurements in layered and/or highly saline soil; PA1 v) signal attenuation inherent in transmission lines, thus precluding the usage of long cables and thereby limiting site coverage or requiring multiple TDR systems for relatively large sites; and, PA1 vi) the inability to reliably measure moisture profiles using a single vertical probe.
Considerable work in soil evaluation and various agricultural applications using Time Domain Reflectometry has been carried out in the past and has been widely reported. See, Ref. Nos. 3, 5-8, 13-16, and 19-26. In the course of that work some improved and excellent transmission line or probe geometries have been developed. Zegelin et al., Ref. No. 28 and International Publication No. W089/12820 based on White et al. International Patent Application No. PCT/AU89/00266.
Measurement of the propagation velocity V of an electromagnetic wave in moist soil as it travels along a transmission line or probe is basic to the TDR soil water content method. Topp et al., Ref. No. 21. Thus, in a typical instance for measuring soil water content, a transmission line or probe having physical discontinuities present at two known locations X.sub.1, X.sub.2 along the line separated by a distance X.sub.2 minus X.sub.1 (where X.sub.2 represents the end of the transmission line and X.sub.1 represents, for example, the coaxial cable/transmission line interface) is imbedded in the soil. The time that a reflection from the discontinuity at point X.sub.1 arrives back at the TDR instrument may be designated T.sub.1, while the time that the reflection from the discontinuity at point X.sub.2 arrives back at the TDR instrument may be designated T.sub.2. Thus, the propagation velocity V is: ##EQU1## As described in Ledieu et al., Ref. No. 14, the propagation velocity V is usually normalized to the speed of light (c) in space using the apparent dielectric constant formula: EQU K.sub.a =(c/V).sup.2 ; [2]
and, since (c) is a known quantity and the propagation velocity V is a measurable quantity, the apparent dielectric constant K.sub.a of the materials surrounding the probe can be calculated to provide, for example, a direct indication of the moisture content of the test material. This is possible because the apparent dielectric constant K.sub.a of moist soils changes substantially as water saturation rises since there is a large contrast in the dielectric constant K of water and that of most dry soil solids. Alharthi et al., Ref. No. 1.
However, one of the most significant problems heretofore encountered in TDR measurement systems resides in the fact that the actual values of T.sub.1, T.sub.2, X.sub.1 and X.sub.2 are extremely critical; and, even relatively small errors can result in significant errors in the calculation of the test material's apparent dielectric constant K.sub.a. Where the TDR system is employed to determine material level in a container, liquid profiles, or the liquid content of the test material, an error in calculating the apparent dielectric constant K.sub.a will, of course, result in a significant error in the ultimate calculated result.
Minimal performance and design criteria for a basic probe to be used with TDR instruments include, for example:
Probes that employ only passive elements cannot meet the foregoing criteria. Excellent examples of such probes include, for example, those disclosed by Zegelin et al., Ref. No. 28 and International Publication No. W089/12820 based upon White et al. International Patent Application No. PCT/AU89/00266.
It is known that the amplitude of the T.sub.1 reflection can be increased by adding passive elements such as reactive components or impedance changes at or near X.sub.1. Ledieu et al., Ref. No. 14; and, Malicki et al., Ref. No. 15. Ledieu et al., Ref. No. 14, employs, for example, two passive diodes in opposition which are soldered to the front ends of the two parallel probe transmission lines adjacent their interface with the coaxial cable. This approach has, however, proven to be highly limited because of the loss of energy at X.sub.1 induced by the two (2) passive diodes which create the T.sub.1 reflection and which serves to significantly reduce the amplitude of the T.sub.2 reflection since the loss occurs twice--i.e., once as the electromagnetic wave is propagated down the coaxial cable/transmission line and a second time as the reflection from point X.sub.2 passes back through the transmission line/coaxial cable to the TDR instrument. This problem is exacerbated by the use of long connecting cables--e.g., cables up to one hundred (100) meters in length or more--having reduced high frequency transmission characteristics. As a consequence, such TDR systems have limited cable lengths and require extremely expensive, high-performance TDR instruments and waveform processors in order to detect the weak T.sub.2 reflection. However, even with an expensive high-performance system, the weak T.sub.2 reflections can, and often do, generate false data.
The use of a strip line probe or transmission line--i.e., a generally one-piece, blade-like, integral probe defined by parallel conductive strips formed on a printed circuit board is described by Fellner-Feldegg, Ref. No. 9. The author suggests covering the two parallel conductive strips with dielectric material to reduce the characteristic impedance of the line.
Experimentation with TDR systems has revealed that the impedance mismatch between coaxial cables and 2-conductor probes introduces an error source into the measurements. It has, therefore, been proposed that a balun transformer be employed to compensate for that impedance mismatch. Dalton et al., Ref. Nos. 5 and 6; Dasberg et al., Ref. No. 8; Nadler et al., Ref. No. 16; Topp, Ref. No. 20; Topp et al., Ref. Nos. 23 and 24; and, Wraith et al., Ref. No. 26. However, the use of a balun transformer not only significantly increases the cost of the system but, moreover, balun transformers are, themselves, a source of error problems. Zegelin et al., Ref. No. 28.
When using 3 or 4-rod probes such as disclosed by Zegelin et al., Ref. No. 28 and in International Publication No. WO 89/12820 based upon White et al. International Patent Application No. PCT/AU89/00266--probes which present significant improvements over other conventional probes in that they are configured to minimize impedance mismatches between the probe and the interconnecting coaxial cable--it has been found that the T.sub.2 reflections can be significantly reduced by physical and/or moisture layers within soil. Thus, reflections are reduced in amplitude by such layering discontinuities which serve to change the impedance characteristics of the transmission line; and, the intermediate reflections caused by the transmission line impedance change at such layered discontinuities become undesirable background noise. This background noise may not only cause false readings but, moreover, may merge with the true T.sub.2 reflection to create significant delay errors. Nadler et al., Ref. No. 16. Indeed, for heavily layered soils, the T.sub.2 reflection can become undetectable by any conventionally employed detection/measurement system.
Topp et al., Ref. No. 22, describes a multiple segment probe wherein the transmission line is designed to produce electrical discontinuities and consequent changed impedance characteristics at known locations along the line by, for example, varying the diameter of the solid brass rods used in the probe at selected points or by employing transmission lines formed of solid polystyrene having spaced areas coated with silver paint and joined by copper tape. However, the intermediate reflections from the electrical discontinuities are even smaller in amplitude than the small natural reflection from the transmission line end; and, consequently, the signal-to-noise ratio is even lower than the conventional T.sub.2 reflection. Field use of this type of probe has not been widely reported in the literature.
Computer-controlled TDR systems have been described in the prior art for making large numbers of soil water content measurements at different sites at predetermined time intervals (Heimovaara et al., Ref. No. 11) and for use with layered soil media (Yanuka et al., Ref. No. 27). Automated and multiplexed TDR systems are described in, for example, Baker et al., Ref. No. 3. In other instances, soil water content has been measured with manually-controlled TDR systems. Malicki et al., Ref. No. 15.
Numerous prior art patents are also available relating to the use of TDR systems for a wide range of applications. For example, U.S. Pat. No. 3,771,056-Zimmerman discloses a display baseline stabilization circuit having a sampling system used to determine the size and location of any discontinuities in the characteristic impedance of the transmission line. The apparatus employs a switch to change the impedance characteristics at the end of the coaxial cable transmission line and thus provide one of several switchable, identifiable impedance changes at the probe terminus.
In U.S. Pat. No. 3,789,296-Caruso, Jr. et al., the patentees describe an apparatus for sensing the moisture content and, therefore, the amount of dielectric coating applied to a web-like carrier as the latter is passed between the sensing bars of a TDR system.
In Wrench, Jr. et al. U.S. Pat. No. 4,109,117, the patentees describe a TDR system in combination with a multiplexing technique for allowing the separation of signals from many transducers on a single coaxial cable passing through multiple sites. In this arrangement it is proposed to use variable impedances equally spaced along a transmission line wherein the variable impedances are in the form of field effect transistors (FETS) or microphones which produce discontinuities in the cable resulting in reflections that are sensed by the TDR system. Wrench, Jr. et al. are not, however, concerned with the measurement of propagation velocity.
In U.S. Pat. No. 4,786,857-Mohr et al., the patentees disclose the use of a TDR system having a coaxial transmission line with a passive terminating resistor to provide an identifiable impedance change at the probe terminus in a fashion somewhat similar to that disclosed in Zimmerman U.S. Pat. No. 3,771,056. The Mohr et al. apparatus is used to determine the relative proportions of intermixed constituents in a multi-phase fluid system.
Malicki et al. U.S. Pat. No. 4,918,375 is of interest for its disclosure of a TDR system for the measurement of soil water content using a Tektronix Model 1502 TDR acquired from Tektronix Corp., Beaverton, Oreg. In this system the patentees employ step-wise, local, specific, passive impedance discontinuities above the air/soil interface to establish a reference time for multiple parallel transmission lines.
Numerous other patent disclosures are of miscellaneous interest in that they disclose other types of systems, bearing certain similarities to TDR systems, for various applications. For example, in U.S. Pat. Nos. 3,853,005-Schendel and 3,995,212-Ross, the patentees insert transmission lines into a liquid container and use the reflection from the air/liquid interface to determine the level of liquid. A somewhat similar arrangement is disclosed in U.S. Pat. No. 4,135,397-Krake wherein the transmission line is inserted into a grain elevator with the transmission line having a passive load impedance Z.sub.L equal to the characteristic line impedance for terminating the transmission line. Again, the reflected pulse from the air/grain interface is indicative of the level of grain.
Wann U.S. Pat. No. 4,949,076 discloses a leak detector employing a coaxial cable used to detect reflected signals from a leak location with the coaxial cable employing a passive terminating resistor.
A somewhat different system is disclosed in Statutory Invention Registration No. H395-Nash wherein the registrant uses a coaxial cable to generate an electrical field at the end of the cable which is attenuated by the electrical characteristics of the material undergoing test; and, the attenuation in the reflected wave is then observed.
Other patents of miscellaneous interest include:
Other prior art patents of general interest which do not relate to either TDR systems and/or to systems for measuring soil water content can be found in the art relating to transmission lines as used in various electronic devices. These include, merely by way of example, Oberbury U.S. Pat. No. 3,757,222 which discloses an RF system, and particularly, a single sideband generator employing diodes which are connected to ground along the length of the transmission line for defining switchable short circuits to advance or retard the phase of the signal at the load in digital steps. Similarly, Bakken U.S. Pat. No. 3,829,796 discloses an electronic amplitude modulator for use in navigational systems using diodes positioned along the transmission line to provide step-wise variation of the phase angle .phi. and, thereby, of the amplitude of the signal voltage.
U.S. Pat. No. 4,349,795-Kwok discloses an amplifier station for the trunk system in cable TV systems wherein a switching apparatus passes RF signals in a prescribed frequency band on a main transmission line to first and second transmission lines. PIN diodes short opposite ends of the first coaxial transmission line for improving isolation of the station equipment.
UK published Patent Application, Publication No. 2,216,355 A-Gale (1989) discloses a voltage-controlled oscillator using PIN diodes soldered to a microstrip transmission line to provide distributed capacitance.
A wide range of other devices have been used to short transmission lines for a wide range of purposes. These include, merely by way of example: i) U.S. Pat. No. 3,551,677-Brewster [a field reversal type pulse generator with a shorting switch formed by a plurality of parallel gas dielectric spark gaps connected across one end of the transmission line--the spark gaps are subjected to ultraviolet light to enable them to break down so as to permit the transmission line to discharge and cause the pulse generator to produce an output pulse]; ii) U.S. Pat. No. 3,993,933-Menninga [an electric overvoltage gas arrester with a metallic shorting mechanism to prevent overheating of a surge voltage gas tube used to protect equipment connected to telephone and other transmission lines]; iii) U.S. Pat. No. 4,755,769-Katz [a composite power amplifier wherein the output of a plurality of amplifiers are combined to produce a higher output signal--shorting switches are coupled to the transmission line and are selectively rendered conductive to adjust impedance and maintain impedance matching]; iv) U.S. Pat. No. 4,782,313-Brant, Jr. [a transmission line shorting switch for preventing transmission of signals along unbalanced transmission lines]; and v), UK Pat. No. 1444540-Heading [an electrical filter which uses a conductive track and wiper assembly on a transmission line to adjust bandwidth].
Notwithstanding the extensive reported work to date relating to TDR in general and specific TDR applications in respect of soil water content measurement, both in the literature and in patents, it has been found that the successful implementation of a TDR soil water content measurement system comprising a commercially acceptable system useful in the field has continued to suffer from numerous practical and/or cost-related limitations such, merely by way of example, as: